Over the past few days I've read an awful lot of speculation about Apple's latest unicorn product—the iWatch. However, much of it seems to be pie-in-the-sky thinking that's not grounded in any form of reality.

I don't know if Apple is working on an iWatch or not, but what I do know is that any smartwatch—Apple or otherwise—is going to have to operate within certain parameters. Let's take a look at some of these constraints.

It has to fit on every wrist. This might seem like a no-brainer, but consider that we are talking about a device that will be equally at home on the dainty arm of a petite girl as it would be on my 8-inch wrist. This consideration alone rules out giant Angry Birds-capable displays.

It has to have decent battery life. No one is going to be happy charging up their iWatch as often as they charge up their iPhone or iPad. But tiny button cells, not massive lithium-ion battery packs are the norm for powering wristwatches. This means that the device has to sip at, as opposed to guzzle from, the battery. Fitting a solar panel to the iWatch would help, but not as much as you think, because unless iWatch owners are willing to walk around with ISS-style panels, we're dealing with tiny amounts of power measure in microwatts. Battery life will need to be measured in days, not hours.

Think Siri, not buttons. Forget about the idea of buttons and apps, and instead think of simple controls and Siri. Remember how much people complained that the 3.5-inch screen on the iPhone 4S was cramped before the iPhone 5? Do you want to walk around with an iPhone 4S strapped to your wrist?

Forget cellular. I've come across several pundits who speculate that the iWatch could take over from the iPhone by having an integrated cellular connection. These people obviously don't know how much power this required. Any iWatch will undoubtedly future wireless, but this will take the form of Bluetooth or Wi-Fi. Cellular is just too much of a battery guzzler.

E-ink makes sense. While it's nice to dream about having a high-resolution retina display fixed to my wrist, the truth is that these screens put far too much pressure on the battery. E-ink is the perfect solution. It's not as sexy as a high-density backlit LCD display, but it means you don't have to be attached to a giant battery.

Think apps, but not iOS apps. Sure, a smartwatch needs apps, but let's be serious here, we're talking apps specifically designed for a low-power device with a small screen. Simple stuff, not Angry Birds and word processors.

Rugged and waterproof. Apple is renowned for making beautiful devices, but they're not the most robust of devices. A smartwatch is going to need to waterproof and tough enough to put up with daily knocks and scrapes. You might be able to stick your iPhone or iPad in a case, but your iWatch is going to have to survive naked on your wrist.

Price, price, price. The Pebble smartwatch retails for $150. Apple's luxury brand status means that it could probably ask around $200 to $250 for the iWatch, but at a 30 percent profit margin, this doesn't leave wriggle room for laser beams and 3D holographic projectors. We're talking a device here based on existing technology, not stuff recovered from the Roswell crash site.